1. Field of the Invention
The present invention relates generally to bicycles and spoked wheels therefor. More specifically, the invention relates to rear tensioned spoked bicycle wheels and hubs designed to have improved and better balanced torque transmission capacity.
2. Description of the Prior Art
Rear bicycle wheels are known as are multi-speed bicycles with rear wheel hubs including a multiplicity of sprockets. In a conventional rear bicycle wheel, with a plurality of sprockets, torque is transmitted through the sprocket to the wheel hub and through flanges on the hub to spokes and through those spokes to the rim. The spokes extend tangentially, relative to the hub flanges, between the hub flanges and the rim. However, under static conditions, the tension in the spokes connected to the hub flange adjacent to the sprockets, hereinafter referred to as the proximal spokes and proximal hub flange, respectively, is substantially greater than the tension in the spokes connected to the hub flange opposite the proximal hub flange, hereinafter referred to as the distal spokes and the distal hub flange, respectively. This disproportionate tension is a consequence of the geometry of such rear wheels. The center plane (CP shown in FIG. 2) of the rim and of the rear wheel intersects the hub between the proximal and distal hub flanges at a point which is much closer to the proximal hub flange than it is to the distal hub flange. In order to achieve this geometry, the lateral forces exerted by the distal spokes on the rim must equal the lateral forces exerted by the proximal spokes on the rim. Since the proximal spokes are more nearly perpendicular to the axis (A shown in FIG. 2) of the hub than are the distal spokes, the proximal spokes must be under more tension in order to offset the lateral forces exerted by the distal spokes on the rim.
Under dynamic conditions, and especially when torque is being transmitted through the hub to the rim, a grossly disproportionate percentage of the torque is transmitted through the proximal spokes to the rim. This phenomenon is a consequence of the flexibility of the hub shaft (designated 13 in FIG. 2) and the greater tension of the proximal spokes, as mentioned above, whereby under static conditions they are stretched closer to their elastic limit which must be approximated before the spokes can transmit torque to the rim. In conventional rear hubs with multiple sprockets, torque is transmitted from the sprockets to the hub at a point or points located generally between the proximal hub flange spokes and the sprockets. Most of the torque applied to a hub having equally sized flanges or to a hub with a larger proximal hub flange is transmitted through the proximal hub flange because the hub shaft (13 shown in FIG. 2) flexibility prevents the torque from reaching the distal hub flange before the proximal spokes achieve their elastic limit, the point at which they begin to transmit torque and because the mechanical displacement, the stretch, of the distal spoke must be greater to get to their elastic limit to compensate for their lower static tension. The torque loss is a function, among other things, of the stiffness of the hub and the distance between the point where torque is applied to the hub and the flange through which the torque is transmitted. In other words, more torque is transmitted to the proximal hub flange than is transmitted to the distal hub flange. Consequently, torque transmission through the hub exacerbates the disproportionate spoke tension which exists under static conditions in conventional rear wheels causing undue and unnecessary strain on the proximal spokes, far in excess of the strain on the distal spokes.
In recognition of the minor contribution of distal spokes in rear wheel torque transmission, Campagnolo, in a wheel having equally sized hub flanges, orients the distal spokes radially so that they transmit virtually no torque to the rim. Virtually all of the torque is transmitted through the proximal spokes. It is hard to imagine a construction which would cause more strain on the proximal spokes. Another bicycle company offers a rear bicycle wheel hub having a proximal hub flange which is substantially larger than the distal hub flange. In virtually every other rear wheel produced by the industry today, there is a hub with equally sized hub flanges. None of these constructions enables the distal spokes to contribute significantly to the transmission of torque from the hub to the rim and tire.
It should be recognized that only half of the tangentially oriented proximal spokes are positioned to transmit driving torque from the proximal hub flange to the rim. The other half are oriented so that they can not transmit torque. As a consequence, one half of the proximal spokes, i.e., one fourth of the total spokes in the rear wheel, wind up transmitting virtually all of the torque to the rim. Since these spokes, under static conditions, are under significantly more tension that the distal spokes, it is easy to understand why nearly one hundred percent of rear wheel spoke breakage occurs in the driving proximal spokes. This characteristic of conventional rear wheels has also thwarted efforts at reducing the spoke count in rear bicycle wheels.